Unravelling the nature of binding of cubane and substituted cubanes within cucurbiturils: A DFT and NCI study

The nature of interactions between the neutral, charged and substituted cubane, within cucurbituril host were analyzed employing the dispersion corrected density functional theory. The structural comparison between the inclusion complexes of CB7 and CB8 shows a best fit for the CB7. The computed Gibbs free energy for the formation of inclusion complexes by the neutral guests were close to the experimental estimate. The dicationic guest within CB has the highest strain energy with least strain on the host. The conceptual DFT based analysis ECT analysis shows a charge transfer from the host to guest in neutral systems, while for the dicationic guest an electron transfer from guest to host has been noticed, which was further conformed from the quantitative MESP values. The computed NICS values on the cubane, are not affected by the introduction of charge/substituents implying that cubane part act as a spacer group while the functional groups present on the cubane dictates the charge transfer. In AIM analysis, the ρ value is least for the complex cubane@CB7 and highest for the cubane functionalized dicationic complex@CB complexes. The NCI-RDG analysis for the inclusion complex with dicarboxylic guest, the spike at higher density region undergoes a shift, reflecting the increase in repulsive energy. In the inclusion complexes of CB7 the patch escalation in the NCI isosurface occurs evenly in CB7 than on the CB8 inclusion complexes, signifying the fit-induced extra stability of the CB7 inclusion complexes. EDA analysis shows that the Pauli repulsive energy increase with the increase in the size of the guest molecule and among the guest the dicationic has the highest repulsive energy. The presence of higher amount of electrostatic interactions in the dicationic complex has remunerated the dispersion contribution in the dicationic complexes.